In a general in-vehicle step-down switching power supply, a step-down ratio is adjusted by controlling a duty (D: ratio (D=Ton/T) of a switching cycle (T) and an on-period (Ton)) of a PWM signal input to a gate of a main switch. Because D is determined by a ratio Vout/Vin of an input voltage (Vin) and an output voltage (Vout), D approaches 100% when Vin decreases and approaches Vout. However, because an on/off-period of a main switch element cannot be made to become zero, from a restriction of securing a gate driving voltage of the main switch by a bootstrap circuit or a minimum on/off transition duration of a switching element, there is a duty allowance range such as 5 to 95% in D. If D is more than a value in the allowance range, a switching power supply may not normally operate. In PTL 1, a method suggested as a method to solve the above problem is described. The method described in PTL 1 changes a switching frequency (F−sw) of a MOSFET according to a voltage generated in the bootstrap circuit to generate a gate driving voltage of the MOSFET, thereby stably driving a switch element.